1. Field of the Invention
This invention relates to a fluid torque convertor which is used in an automatic transmission, in detail, relates to a control device for a torque convertor.
2. Description of the Prior Art
In general, a control device of a lock-up clutch for a torque convertor is divided by a clutch disc (plate) into two sections: the first oil chamber (clutch-off pressure) of one side and the second oil chamber (clutch-on pressure) of the other side. The clutch is connected by the oil pressure from the second oil chamber to the first oil chamber while, on the other hand, the reverse oil supply from the first to the second releases the clutch. The amount of a differential pressure of both the oil chambers is controlled by external signals such as the vehicle speed and the opening ratio of the engine throttle so that the engaging force of the clutch is controlled and regulated. Conventionally, as shown in the Japanese Patent Publication "Tokuko-Sho 63-5625", the first and the second oil chambers connect through an orifice. Furthermore controlling chambers connected to both the first and the second oil chambers are provided in the opposite position and controlled by external signals. A lock-up clutch control has a draining oil passage from the first oil chamber and a regulating valve is provided.
This lock-up control device directly senses the differential pressure by applying the inside pressure of the first and the second oil chambers to the regulating valve. Therefore the regulating valve is controlled correctly according to the external signals and can control the clutch engaging force regardless of oil temperature change.
Generally, a torque convertor whose outside casing is made of a relatively thin steel plate expands or contracts with changes in the centrifugal force and the supplying pressure, and its configuration changes. At the mentioned regulating valve (hereinafter a lock-up controlling valve) the supplying oil pressure of the first oil chamber is led to the second oil chamber through the orifice, and furthermore, as the draining oil pressure (off pressure) from the second oil chamber is regulated and controlled at the required moment, the draining oil pressure to be the origin of this control may not be able to respond to the change of the configuration of the torque convertor, because the volume of the pressure through the orifice is comparatively small. For example, when the vehicle speed decreases from a high speed condition and slipping is allowed between the transmission input and output by decreasing clutch engaging force, the draining oil pressure from the first oil chamber needs to be increased. In this state, as the volume of the torque convertor contracts based on the decrease of the centrifugal force and of the supplying pressure, the oil pressure in second oil chamber increases. Therefore torque fluctuation may not be able to be absorbed because only controlling only the amount of drainings from the first oil chamber through the orifice is not enough and because the engaging force is too strong.
At the lock-up control valve, as a governor pressure (or a throttle pressure) which is regarded as an external signal is controlled, the transient control at engaging and releasing is not enough and controlling the lock-up clutch continuously under slipping condition is difficult.
Furthermore, if the lock-up control valve sticks and the draining oil pressure (off pressure) is kept high, not only the lock-up clutch but also the torque convertor slips and overheats. At this state an oil circulation of whole the torque convertor is stopped and overheating results.